Selective Colorimetric Detection of Nitrite in Water using Chitosan Stabilized Gold Nanoparticles Decorated Reduced Graphene oxide

Excess nitrite (NO2 -) concentrations in water supplies is considered detrimental to the environment and human health, and is associated with incidence of stomach cancer. In this work, the authors describe a nitrite detection system based on the synthesis of gold nanoparticles (AuNPs) on reduced graphene oxide (rGO) using an aqueous solution of chitosan and succinic acid. The AuNPs-rGO nanocomposite was confirmed by different physicochemical characterization methods including transmission electron microscopy, elemental analysis, X-ray diffraction, UV-visible (UV-vis) and Fourier transform infrared spectroscopy. The AuNPs-rGO nanocomposite was applicable to the sensitive and selective detection of NO2 − with increasing concentrations quantifiable by UV–vis spectroscopy and obvious to the naked eye. The color of the AuNPs-rGO nanocomposite changes from wine red to purple with the addition of different concertation of NO2 −. Therefore, nitrite ion concentrations can be quantitatively detected using AuNPs-rGO sensor with UV-vis spectroscopy and estimated with the naked eye. The sensor is able to detect NO2 − in a linear response ranging from 1 to 20 μM with a detection limit of 0.1 μM by spectrophotometric method. The as-prepared AuNPs-rGO nanocomposite shows appropriate selectivity towards NO2 − in the presence of potentially interfering metal anions.

In recent years, metal nanoparticles have gained immense attention in various disciplines due to their unique physicochemical properties in terms of large surface area 1 , excellent adsorption characteristics 2 and high electro-catalytic activity 3 . The unique properties of metal nanoparticles render them suitable to applications across a range of disciplines including catalysis 4 , chemical sensing 5 , biolabeling 6 and photonics 7 . In particular, gold nanoparticles (AuNPs) have shown superior properties when compared to other metallic nanoparticles [8][9][10] . Due to their high molar extinction coefficient 11 , strong localized surface Plasmon resonance 12 , distance dependent optical properties 13 , stability 14 , and strong, well-defined color color change 15 , AuNPs have been widely applied for the selective probe for detection of anions 16,17 .
Among the three-inorganic nitrogen-containing nutrients (NH 4 + , NO 2 − , and NO 3 − ), nitrite (NO 2 − ) is essential nutrients for the growth of plants 18 . Nitrites (NO 2 − ) are widely used for the preservation of food and curing of meat, with an acknowledged toxicity through the excessive uptake of NO 2 − 19 . In humans, high concentrations of NO 2 − are associated with a number of medical issues such as methemoglobinemia, gastric cancer, and hypertension due to formation of carcinogenic N-nitroso compounds 19,20 . Moreover, NO 2 − reacts with oxyhemoglobin in the blood and causes methemoglobinemia 18 . The World Health Organization set the fatal dose of NO 2 − as 1.0 mg/L 21 . Given the reactivity of NO 2 − , a method for the rapid and accurate detection of NO 2 − has great potential to help reduce the associated health risks. Analytical techniques developed for determination of NO 2 − include spectrophotometry 22 , electrochemistry 23 , fluorescence 20 , chromatography 24 , chemiluminescence 25 and surface-enhanced Raman scattering 26 . However, these methods have limited application in the routine detection of NO 2 − due to a dependency on expensive reagents, instrumentation, long duration incubation periods, and highly skilled operators. Nonetheless, the sensitive, selective determination of NO 2 − utilizing these technologies represents a significant challenge.
Due to its simplicity, high sensitivity, selectivity, easy operation, cost effectiveness and fast response 27 , colorimetric detection is widely applied to the trace level detection of NO 2 − . Recently, metal nanoparticles have been widely used for colorimetric detection of NO 2 − . As an alternative, reduced graphene oxide (rGO) possesses large specific surface area, and extraordinary mechanical, thermal, and electrical properties 28 . Owing to its unique properties, rGO has served as an excellent support for synthesis of nanoparticles decorated rGO or graphene composites. Furthermore, different chemical and electrochemical reduction methods have been used for the synthesis of metal nanoparticles decorated graphene composites including AuNPs-rGO composite. In addition, AuNPs decorated rGO in the presence of chitosan have also been well documented [29][30][31] . In the present work, we have synthesized chitosan stabilized AuNPs-rGO nanocomposite in presence of succinic acid for the first time ( Fig. 1) and applied this to the colorimetric detection of NO 2 − . The selectivity and practicality of the NO 2 − sensor has been critically studied and discussed.

Results and Discussion
Characterizations of AuNPs-rGO composite. The AuNPs-rGO composite were synthesized using chitosan and succinic acid as a reducing agent at 60 °C. Initially, GO was combined with chitosan to form a GO/ chitosan composite. Succinic acid was utilized as a reducing and stabilizing agent for synthesis of the AuNPs-rGO composite. The reduction rate of AuCl 4 − is enhanced by the presence of chitosan due the increase of the electrostatic interaction between NH 3 + and AuCl 4 − due to the higher degree of protonation in the amino groups of chitosan. A schematic representation for the synthesis of chitosan stabilized AuNPs/rGO composite is shown in Fig. 1.
The formation of the AuNPs/rGO composite was confirmed by UV-vis spectroscopy. Figure 2 displays the UV-vis spectra of a) GO, b) AuNPs and c) AuNPs− rGO. UV-vis spectrum of GO shows 2 maximum bands at 249 and 344 nm, which are due to the π → π* transition of aromatic C-C bonds and n → π* transition of C = C 30 . UV-vis spectrum of AuNPs shows a sharp absorption maximum at 525 nm due to the surface Plasmon resonance of AuNPs 30 . UV-vis spectrum of AuNPs-rGO shows 2 maximum bands with red shifts at 536 and 266 nm, which are due to transition of aromatic C-C of rGO and surface Plasmon resonance of AuNPs 32 . The results confirm formation of the AuNPs-rGO composite.
The formation of AuNPs-rGO composite was further confirmed by Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). Figure 3 shows the FTIR spectra of a) GO and b) AuNPs-rGO composite. The FTIR spectrum of GO shows the broad and intense absorption peaks at 3435, 1723 and 1245 cm −1 corresponding to the stretching vibrations of -OH, -C = O (carbonyl) and C-O (epoxy) groups, respectively 33 . Conversely, the characteristic absorption peaks of -OH, -C = O in the AuNPs-rGO composite decrease dramatically, indicating the successful transformation of GO to RGO 33 . Figure 4 shows the XRD profiles of a) GO, b) rGO, c) AuNPs and d) AuNPs-rGO composite. The XRD of GO showed a sharp diffraction peak at 2θ = 11.20, suggesting the complete exfoliation of graphite. XRD of rGO    AuNPs on AuNPs-rGO composite was calculated using the Scherer equation and indicates an average grain size of 25.6 ± 3 nm. The above results further support the formation of AuNPs-rGO composite.
The surface morphologies of as-prepared AuNPs-rGO composite were investigated by transmission electron microscopy (TEM). Figure 5 displays the TEM images of a) rGO and b) AuNPs-rGO. It can be clearly seen that the spherical AuNPs are uniformly dispersed on the surface of rGO with an average diameter of 26 nm. Additionally, the TEM image of rGO shows the laminar structure with the association of few layers of nanosheets. The average diameter of AuNPs (26 nm) in AuNPs-rGO composite is in good agreement with the XRD results. As shown in Fig. 5c, the elemental analysis (EDS) confirms the presence of carbon, oxygen and metallic gold in AuNPs-rGO composite. The above results validate the formation of AuNPs-rGO composite.
To optimize the loading of chitosan, the AuNPs-rGO composite was prepared by the addition of 0.1 g (A), 0.3 g (B), 0.5 g (C) and 0.7 g (D) chitosan and corresponding scanning electron microscopic (SEM) images are shown in Fig. 6. The SEM images of AuNPs-rGO composite clearly reveals that 0.5 g addition of chitosan shows a more uniform morphology than the 0.1, 0.3, or 0.7 g. Hence, 0.5 g chitosan is considered the optimum for synthesis of AuNPs-rGO composite.
Raman spectroscopy is an ideal technique to confirm the transformation of GO to rGO and the representative Raman spectra of rGO (red line), GO (green line) and AuNPs-rGO (blue line) are shown in Fig. 7. The Raman spectrum of GO (green color) shows distinct D and G peaks at 1344 and 1583 cm −1 , which are attributed to the vibrations of sp 3 and sp 2 carbon atom domains of graphite. The intensity ratio of I D /I G was higher in rGO (1.02) and rGO-AuNPs (1.05) than GO (0.92), which clearly shows the transformation of GO to rGO.

Colorimetric determination of NO 2
-. To demonstrate the capacity detect and quantify low levels of NO 2 − , UV-vis spectroscopy was used to determine the concentration of NO 2 − in an aqueous solution. Figure 8a shows the UV-vis absorption spectra of AuNPs-rGO for the addition of different concentration of NO 2 − (0-200 µM) into the AuNPs-rGO composite solution. As can be seen, the peak at 525 nm was shifted and a new peak appeared at 626 nm upon addition of NO 2 − . An increase of NO 2 − concentration from 0.1 µM to 200 µM resulted in an increase of absorbance in the 626 nm region increased and a corresponding decrease in absorbance in the 525 nm region (Fig. 8b). The value of A685/A520 increased linearly with a lowest detection capability of 0.1 µM. The calibration curve for the value of A685/A520 vs. [NO 2 − ] was linear in the detection range from 1 to 20 µM with a correlation coefficient (R 2 ) of 0.998 (Fig. 8c). The lowest detection level of our sensor is well below for the maximum level of NO 2 − in drinking water (21.7 µM) as set by the Environmental protection agency (EPA). It was noted that the results confirmed the proposed material as highly suitable to real-time, on-site detection of NO 2 − in drinking water. It was also noted that the analytical performance of AuNPs-rGO nanocomposite is superior towards NO 2 − than AuNPs prepared in the absence of rGO (data not included). The obtained analytical results of our sensor were compared with the previously reported sensors, and the analytical results gained (LOD and linear response range) are comparable with reported nitrite sensors including electrochemical systems 19,20,23,27,[34][35][36] . It also noted that the analytical performance of sensors is more comparable to previously reported colorimetric detection of NO 2 − based on gold nanomaterials (AuNPs and nanorods) [37][38][39][40] . Accordingly, the present sensor demonstrates practical potential for the sensitive detection of nitrite at low levels.     AuNPs-rGO, arising from a closer formation of the metal nanoparticles. The presence of chitosan enhances the stability of AuNPs-rGO in aqueous solutions. The results demonstrate that the proposed method can be used for the selective direct detection of NO 2 − both with the naked eye, and with spectrophotometric methods.

Selective detection and mechanism of NO
Determination of NO 2 in water samples. To evaluate the practical ability of the sensor, the concentration of NO 2 − was determined for a range of collected water samples by UV-vis spectrophotometry. Three different tap water samples were collected from the Thiagarajar college campus, Madurai and all obtained water samples were shown to be NO 2 − free. A known quantity of NO 2 − (5 µM) was added and the water samples were re-analyzed. The concentration of NO 2 − in the spiked water samples was calculated from the linear plot of A626/ A525 as shown in Fig. 8c, and the obtained recoveries tabulated in Table 1. The average recovery of NO 2 − in the adulterated water samples was determined as 97.0% with a relative standard deviations (RSD) of 5.0% (n = 5). The results demonstrate the potential application of the proposed AuNPs-rGO sensor in the detection of NO 2 − in water and food samples.
The stability and reproducibility of the sensor was also evaluated under the experimental conditions outlined in Fig. 8a. The AuNPs-rGO/CS composite shows appropriate stability (~98.2%) through continuous monitoring over a period of 14 days (figure not shown). In addition, the fabricated sensor shows appropriate reproducibility with an RSD of 5.3% across five independent sensors in the detection of 5 µM NO 2 − (figure not presented). Accordingly, the fabricated AuNPs-rGO/CS composite is suitable for long duration and accurate detection of

Conclusions
In conclusion, a novel and selective colorimetric sensor has been developed for determination of NO 2 − using AuNPs-rGO composite as a colorimetric probe. The as-prepared nanocomposite has been thoroughly characterized and the obtained results confirmed the formation of AuNPs-rGO composite. Under optimized conditions, the lowest detection level of our sensor is 0.1 µM and is well below for the maximum level of NO 2 − in drinking water (21.7 µM) that set by the Environmental protection agency (EPA). The sensor also showed high specificity towards NO 2 − in the presence of range of metal anions. The sensor showed acceptable recovery towards NO 2 − in water samples, which authenticates its potential real time sensing ability. We believe that as-prepared chitosan stabilized AuNPs-rGO composite represents a simple, robust, inexpensive material with great potential for application in the sensitive and low-level detection of NO 2 − .

Experimental
Materials and Methods. Fine graphite powder (<50 μm) was received from Sigma-Aldrich, India.
Tetrachloroauric (III) acid trihydrate, chitosan, sulfuric acid (AR grade), potassium permanganate, hydrogen peroxide (30%), sodium nitrate, sodium nitrite, sodium fluoride, sodium bromide, sodium thiocyanate, calcium oxalate, disodium phosphate, sodium bicarbonate, sodium sulphate, and succinic acid were obtained from Merck,  India. All chemicals were of analytical grade and used as received. The stock solutions were prepared using doubly distilled water and the experiments were performed under ambient conditions. UV-vis spectral measurements were performed using a Jasco (V-560) spectrometer. The morphological studies of the as-synthesized composite were characterized by FEI Tecnai G2 20 S-TWIN TEM with an accelerating voltage of 200 kV. FEI Tecnai G2 20 S-TWIN TEM attached BRUKER AXS elemental analyzer was used for the EDS and elemental mapping of the composite. XRD analysis was performed using from Panalytical X′ per PRO X-ray diffractometer equipped with Cu Kα radiation (λ = 0.15406 nm). FTIR was performed by a Shimadzu model FT-IR spectrometer.
Synthesis of GO. GO was synthesized from natural graphite based on Hummers method 30 , with some modification. Briefly, 2 g of graphite powder was added into the mixture of 1 g of NaNO 3 and 50 mL H 2 SO 4 and stirred for 30 min at 0 °C. Subsequently, about 12 g of KMnO 4 was slowly added into the above mixture with continued stirring for 2 h. The temperature of the mixture was then heated to 35 ± 5 °C for 30 min. Approximately 150 mL of water was slowly added to the mixture and the solution was heated to 90 °C with vigorous stirring for 15 min. Then, 120 mL of H 2 O 2 aqueous solution was added to the suspension until its color was changed to brilliant yellow. The obtained graphite oxide was washed three times with diluted HCl (5%) and doubly distilled water, and dried in a vacuum oven at 50 °C for 24 h.

Synthesis of AuNPs-rGO composite.
To synthesize AuNPs-rGO composite, 0.5 g of chitosan was added to the 50 mL of GO suspension (1 mg mL −1 ) and the mixture was stirred for 30 min. Simultaneously, 0.1 M of succinic acid solution and 50 mL of HAuCl 4 (1 mM) solution was added. The mixture was heated to 60 °C with reflux under magnetic stirring until the color turned to wine red. The rGO and AuNPs were also prepared using similar method without AuNPs and GO. The as-synthesized AuNPs-rGO composite was dried in an air oven.